I have Fusion stereo with a 4" Fusion speaker on each side of my console. In researching, I ran across the M and the MX series, not sure what the differences are. I want to replace the current speakers( one rattles) with JL 6.5" marine speakers. What speaker should I use that will not require a dedicated enclosure. I don't have an amp, and I don't want to be heard across the lake, I just want a decent sound at a decent volume.

Most 6.5" speakers are designed for infinite baffle installation... In other words, no enclosure needed. That said, it does need a solid, leak free, mount that isolated the front of the speaker from the rear of the speaker.

Also, aside from the materials used, there is no real difference between "marine" speakers and most automotive speakers. As long as the automotive speaker doesn't use any steel baskets of paper cones, it will work as well as a "marine" speaker. However, some insurance companies could deny a claim on a marine
Vessel IF the cause of damage could be proven to be related to the "non marine" product being used.

I think just about any JLAudio speaker will work fine in a boat they all have powder coated baskets and polypropylene cones.

I picked up a pair of JL 6.5s yesterday. They weren't cheap but quality gear never is. JLs were what everyone I talked to recommended so there must be something to it. I havn't cheaped out rigging this boat so far so there's no reason to start now. The Fusion speakers that I took out of a T-top electronics box on my other boat were [censored].